Artificial turf athletic mats for use for golf practice, or on a golf driving range, desirably have a particular set of physical properties. First, it is desirable to utilize a grass-like surface to simulate a grass golf course natural turf and surface. Second, the underlying support for such a mat is very important because golfers swing long, relatively heavy clubs, with great force. Should the club head ground on an unyielding surface, the surface, the club, and the golfer, may be injured. Third, on practice ranges, the surfaces of golf mats are subject to great wear because of the high level of use and frequent abuse. Previously disclosed mats have been found to be less than entirely satisfactory because they do not effectively simulate natural conditions and they wear too rapidly when in use.
Golf tee shots require the golf club to be swung so that the head passes cleanly through the position of the ball without impacting the ground surface. Practice surfaces used at driving ranges for tee shots should, however, deform to allow passage of the club head should the golfer strike the ball inexpertly and the club head inadvertently make contact with the practice surface. The surface should remain substantially undamaged so that a golfer may put in the hours of practice needed to obtain the desired level of proficiency. Previously disclosed mats have not satisfactorily met this need.
Golf fairway shots require the golf club to be swung so that the head passes through the position of the ball and on into the ground surface so as to displace a portion of turf, a "divot." Practice surfaces for fairway shots should deform to allow passage of the club head, provide sufficient resistance to give the "feel" of taking a divot, and remain undamaged so that a golfer may put in the hours of practice needed to obtain the desired level of proficiency.
Natural turf has a structure that can be described as four general layers. Each layer has properties that affect the performance of the overall playing surface. The layers differ by function and the type of material they have in them. The layers are described herein as the grass layer, the vegetative layer, the root-biomass layer and the root-soil layer. The grass layer consists largely of the grass blades on which the golf ball lies when in play. The primary variable characteristic of this layer is the length of the grass blades. On a golf course fairway, grass is generally maintained at a uniform level that varies depending upon the species of grass used on the course. For example, "Bent grass" courses are maintained at about 0.6 cm (1/4 inch) and rye grass courses at about 1.25 cm (1/2 inch). In the rough, the grass blade length may vary widely as discussed below.
The vegetative layer is the layer where the grass blades join into the main vegetative meristem ("stem") of the grass plant and is the region from which the plant grows. The vegetative layer is thin, firm, springy and supportive. The support is formed by the connections between the grass blades of the first layer, the stems of the second layer and the roots of the third layer. The vegetative layer is generally only 0.3 cm (1/8 inch) to 0.6 cm (1/4 inch) high with stems about 2 mm (3/32 inch) in diameter depending on the grass species and the mown height of the grass. The overall height of the vegetative layer is directly proportional to the grass blade length, e.g., in 5 cm (2 inch) high rough the vegetative layer might be 1.25 cm (1/2 inch) high and composed of stems 0.45 cm (3/16 inch) thick. Such rough offers substantial resistance to the passage of a club head. If grass grows past 1.9 cm (3/4 inch) in height, it begins to form a thatch, a build-up of interwoven live and dead grass, which increases the depth and density of this layer to an extent unsuitable for golf. Thatch is one reason why rough is so difficult to play. Keeping the grass 1.25 cm (1/2 inch) or less in height prevents the development of thatch and maintains direct access to the root-soil layer.
The structural integrity of the vegetative layer of the fairway offers little resistance to the passage of the club head because of the lack of thatch. The club head has sufficient mass and speed, and also has a sharp leading edge that enables it to cut through the turf. The integrity of the vegetative layer provides a vertical support matrix that holds a divot together and maintains the structure of the turf surrounding the divot until it is replaced. Without this structure a divot would fragment into its individual components. Dynamically, the divot flies just after and along the same relative path as the ball until air resistance slows it down.
The root-biomass layer of natural turf is extremely complex and is composed of the grass roots, living and dead organic material, mineral particles, air, water, and a variety of living organisms. The activities of the living organisms of the root-biomass layer (the biomass) constantly work to break down minerals and other materials to particles of critical size. The action of living organisms has a significant effect on the health of soil and plant life, and also has important effects on the performance characteristics of the turf.
On a golf course the root-biomass layer is usually about 50-percent space and 50-percent solid material. Grass roots help living organisms break up the soil and allow air and water to penetrate into the ground. Roots also establish vertical and horizontal integrity. They grow down to access water and out to increase their surface area for absorbing nutrients. The natural growth of roots gives turf its ability to repair itself. The 50-percent of space is about half filled with water which forms a thin film over almost all particles and materials in the layer. Through the playing season decaying material builds up, and constant activity compacts the turf. Golf courses aerate the turf twice a year to remove a percentage of the solid material so as to alleviate compaction and maintain the 50/50 ratio. Below the first 5 cm (2 inches) the amount of space decreases with increasing depth.
The grass roots provide a structural integrity that holds the turf together in the face of natural forces and that holds a divot together when it is displaced by stroke of a golf club. In a divot struck from natural grass, the soil stays trapped in the roots of a divot so that turf rapidly regrows when the divot is replaced. However, the degree to which the other materials are present affects how a club head will cut through the soil. The root-biomass layer is usually 2-3 cm (an inch or so) thick, depending on the grass species and the soil type. The horizontal growth of roots forms a matrix throughout the biomass layer that keeps the soil consistently loose and elastic. Most horizontal root growth occurs in the top 3.8 cm (11/2 inches) and takes advantage of the nutrients present in the biomass layer. Roots penetrate significantly into the fourth layer. In summer, healthy grass roots grow 20-35 cm (8-14 inches) or more into the soil, depending on the species, the health of the soil, and the availability of water. Roots help link the third and fourth layers of the soil together and increase the flexibility of the root-soil layer. The third and fourth layers are also linked together by the physical interactions of the inorganic materials.
The fourth, or root-soil layer, consists of the soil below the horizontal roots of the root-biomass layer. Other than the roots, this layer is compositionally similar to the third layer. However, incidence of living organisms declines with depth and the density of the soil increases with depth due to increased packing of the soil. The root-soil layer provides a firm base that supports the flexibility of the top three layers. The root-soil layer extends indefinitely below the surface, but meaningful texture and air infiltration is found only in the upper 5 cm (2 inches) of the root-soil layer.
The grass on golf courses is deliberately cut to different lengths on different regions of the course. On fairways, the grass is usually cut to 1.25 cm (1/2 inch) or less in height. This short grass makes playing conditions uniform and, permits more consistent play because the variability of the surface is diminished. The grass on greens and tees is usually cut to 0.6 cm (1/4 inch) or less in height. The player has a great deal more control over the ball when the ball is so close to the ground. However, while the effect of the grass is diminished, the soil still has a significant role in affecting play.
The "rough" of a golf course is of various lengths, generally no shorter than 2.5 cm (1 inch) and up to lengths of 7.5-10 cm (3-4 inches). A ball that is sitting down in deep rough, i.e. rough that is higher than 5 cm (2 inches), is very difficult to hit. Deep-rough grass is long and thick. It wraps around the club head and impedes its path to the ball. The ball will likely fly only a short distance compared to the distance the ball would fly if struck from the fairway. When the ball lies in short rough, several ball flights are possible, and the golfer has little control over the distance in this situation as well. For example, if the grass acts as a tee, the ball sits up and a player could hit the ball much further than intended. Practice tee areas are cut to fairway height to make practice more consistent for the player.
In areas such as deep rough or a bunker, the golf club cannot cut through very well because there is just too much resistance. However, bunkers are made of sand, a type of soil preparation which is uniform and consistent in its reactive qualities. Skilled players can use their skill to hit accurate, consistent shots from bunkers.
Golf players deliver force to the surface in two ways. As the player makes a stroke, turf is impacted in two places. First, as the club head strikes down and through the turf surface and strikes the ball and natural turf, the force of the impact radiates out and down about 5-7.5 cm (2-3 inches). Second, the cleats of the shoes of the player create a shearing force on the turf as they push and twist through the stroke. The action of the golfer's feet can cause substantial wear because at a practice area, the player stands and hits from one position.
In golf, force is delivered to the playing, or "striking," surface by the impact of the club head. Golf balls and golf clubs are designed to propel balls with backspin in order to affect and control the distance and direction of the flight of the ball. The club is designed to trap the ball between the face of the club and the ground so that the club face can exert a greater spin force on the ball. Forces are applied to a golf surface by the ball as it is trapped between the club head and the surface and by the club head directly. Forces are applied by the head as it swings through the line of travel, such forces may be delivered at speeds from about 100 cm/sec (a few miles per hour) up to 6700 cm per second (150 mile per hour) with compression loads from almost zero up to 140 Kg per square cm (2,000 pounds per square inch). Force is also delivered as the club head rotates about the axis of the club shaft. Shear forces may occur at up to 30.degree. from the direction of application of the direct force due to the rotation of the club shaft and the curvature of the club head path. The properties of the surface therefore have a great effect on results achieved with the club.
Natural turf practice areas are not readily available in urban areas, and many artificial turf devices have been developed over the years in an attempt to accurately simulate the feel of natural turf to the golfer. Previously disclosed artificial golf mats have portions that can move in response to the impact of a club head. Such mats use springs, rubber bands, or the like to provide a movable surface. Other golf mats have artificial turf surfaces made of belts that move along the path of travel of the club head. These devices have achieved some success, but in general have proved too complex, too unreliable, or too cumbersome or non-portable for regular golf use.
Other prior art discloses mats for golf that are a simple rubber mat, some embodiments having a pile surface and others having a textured rubber surface. Polyurethane has also been used to form a golf mat, usually by being adhered to the back of a tufted carpet material. Such arrangements have not been found acceptable because the polyurethane pad and the tufted layer easily de-laminate, and the entire mat often fails to withstand the blows from the club head and rips into pieces.
Still other disclosed golf practice mats comprise an artificial turf surface bonded to a base formed of foam rubber or other multilayered materials. These devices are less complex than those having movable portions, but again have not proven totally successful because they do not accurately simulate the feel of natural turf to the golfer and tend to de-laminate along their edges or internally as the layers of which they are composed become separated. Other mats that have been made sufficiently strongly to avoid delamination are rigid and provide an unnatural feel when used.
Golf mats conventionally used at driving ranges have a very short lifetime because of the damage done to the mat in the area of the tee. Golfers practicing their tee shots will often hit the mat with a club rather than cleanly hit the ball off of the tee. Previously disclosed mats cannot withstand the substantial forces received by the mat under these conditions and become locally worn beyond use.
Previously disclosed golf mats have tended to be vulnerable to weathering. Exposure to ultra-violet light, heat from the sun and water from rain affect the long term quality of any surface. The artificial materials previously disclosed have proved unable to resist the radiation and heating effects of the sun. Open-celled foams that have been previously disclosed have a tendency to absorb water when exposed to the elements. All foams suffer from hydrophilic degradation. Open-celled foams, however, absorb and hold moisture that promotes degradation.
Because the many previously disclosed golf mats are multilayer constructs, their novel aspects primarily involve the bond between an artificial grass surface and a supportive substrate.